Chemical process of decomposition

About: Chemical process of decomposition is a research topic. Over the lifetime, 1124 publications have been published within this topic receiving 19145 citations.

Calorimetric process monitoring of thermal decomposition of B–N–H compounds

Abstract: Borazane BH3NH3 is a crystalline solid with a high hydrogen content. It decomposes thermally activated already at temperatures below 410 K. The thermal decomposition of BH3NH3 was studied by thermogravimetry (TG), differential scanning calorimetry (DSC), volumetric measurements and coupled TG/FTIR. Measurements were performed under isothermal conditions and in scanning mode. The enthalpy change at the exothermic decomposition reaction ΔrH was determined by use of DSC-technique. At different heating rates and temperatures a value of ΔrH = −(21.7 ± 1.2) kJ/mol BH3NH3 was obtained. It can be pointed out that under appropriate conditions borazane decomposes completely below the melting temperature of T = 385 K given in the literature. As a consequence of the low decomposition rate a separation of different steps is possible only at low heating rates. The decomposition reaction is accompanied by hydrogen evolution. During this first decomposition step borazane releases approximately 1 mol H2 per mol BH3NH3. The other decomposition products are a solid residue of polymeric aminoborane (BH2NH2)x and a small amount of the volatile borazine B3N3H6. The solid aminoborane was characterised by X-ray powder diffraction measurements, IR-spectroscopy and elemental analysis. The small amount of borazine formed was detected by the coupled TG/FTIR-investigations. The mass of the hydrogen released below T = 385 K is about 6.5% of the initial sample mass. Due to the significant amount of evolved hydrogen and the exothermic character of the decomposition process the use of borazane as a source for hydrogen seems to be possible and interesting.

Hydrogen production from water utilizing solar heat at high temperatures

Abstract: Possibilities of producing hydrogen and oxygen from water utilizing solar heat at high temperatures are investigated. The process of direct thermal decomposition of water is studied using a conceptual model. It is shown that the thermodynamic requirements for the direct thermal decomposer are difficult to realize from the structural viewpoint and that existing separation methods are not applicable for such a decomposition process if it is to attain sufficiently high thermal efficiencies. Feasibilities of realizing simple two-step thermochemical decomposition processes are investigated based on existing thermochemical data. It is predicted, as the results of thermochemical as well as thermodynamic analyses, that a two-step thermochemical decomposition process using iron oxide operates efficiently at relatively low temperatures attainable with solar heat and compatible with structural materials.

The effect of the biomass components lignin, cellulose and hemicellulose on TGA and fixed bed pyrolysis

Abstract: Thermochemical conversion of biomass has been studied extensively over the last decades. For the design, optimization and modeling of thermochemical conversion processes, such as fixed bed pyrolysis, a sound understanding of pyrolysis is essential. However, the decomposition mechanism of most biomass types into gaseous, liquid, and solid fractions is still unknown because of the complexity of pyrolysis and differences in biomass composition. The aim of this study was to find characteristic differences in the pyrolysis behavior of three widely used biomass feedstocks to optimize the performance of industrial fixed bed pyrolysis. This aim was achieved in three steps. First, devolatilization kinetics during pyrolysis of three biomass types was investigated in a thermogravimetric analyzer (TGA). Then, a one-step multi-component pyrolysis model with three independent parallel reactions for hemicellulose, cellulose and lignin was derived to correlate the kinetics with single component decomposition and to identify their amount in the biomass sample. In a final step, the findings were tested in a fixed bed reactor at laboratory scale to prove applicability in industrial processes. Three types of biomass were chosen for this investigation: wheat straw, rape straw and spruce wood with bark. They represent biomass with a high cellulose, hemicellulose and lignin content, respectively. Since lignin is the most stable and complex of these three biomass components, its amount is assumed to be the main controlling factor in the thermochemical decomposition process. The thermogravimetric (TG) curve of spruce wood with bark was found to shift to about 20 K higher temperatures compared to the TG curves of straw and rape straw. This result indicates that a higher activation energy is needed to decompose woody biomass, which contains a higher amount and a different type of lignin than straw. Three wood decomposition phases were distinguished from the negative first derivatives curves (DTG): a shoulder during hemicellulose decomposition, a peak during cellulose decomposition and a smaller rise during lignin decomposition. By comparison both herbaceous biomass types decomposed in only two phases at lower temperatures. The decomposition of the herbaceous, and woody biomass samples was completed at about 830 K and 900 K, respectively, leaving only a solid residue of ash. The derived pyrolysis model estimated the composition and described the devolatilization curves of each biomass with sufficient accuracy for industrial processes, although the same activation energy set, taken from the literature, was used for each biomass. In the fixed bed pyrolysis experiments similar characteristics were found to those in the TGA experiments. Herbaceous biomass with a higher cellulose and hemicellulose content decomposed faster and produced a larger fraction of gaseous products than woody biomass with a higher lignin content. According to the assessment of the product distribution, performed after each experiment, woody biomass pyrolysis led to a larger fraction of solid products than herbaceous biomass pyrolysis. We conclude that industrial fixed bed pyrolysis can be optimized for different biomass feedstocks with a specific composition of cellulose, hemicellulose and lignin.

The reflectance spectra of organic matter in the visible near-infrared and short wave infrared region (400-2500 nm) during a controlled decomposition process

Abstract: The reflectance spectra of organic matter in the VIS-NIR-SWIR regions (400–2500 nm) were investigated with regard to possible changes that might occur during a biological decomposition process. Two different groups of organic matter were used in this study: a grape mart (CGM) and a separated cattle manure (CSM) that simulated pure organic matter endmembers in soils. Exposing the two materials for different decomposition durations (0–378 days) visually yielded color sequences as the compost aged. Significant changes in the reflectance spectra of both materials were also observed during the composting period, which provided parameters for controlling the composting process. The slopes in the VIS-NIR region were found to be basic parameters for monitoring changes and were found to be highly correlated with other chemical parameters often used for assessing organic matter conditions in the field (such as the CIN ratio). It was found that during the initial composting stage (0–60 days) the slope parameters were strongly affected by the decomposition activity and, hence, errors in the assessment of organic matter content of soils using slope (or band ratio) parameters are likely. Careful observation of the major spectral features reveals that the reflectance spectrum in the VIS-NIR-SWIR region is a very sensitive tool for monitoring slight changes. Application of the near-infrared analysis (NIRA) pathways revealed that OH and CH groups combined with hygroscopic water, starch, cellulose, and lignin are the components having the highest correlations with composting time within the conditions used. Because of the small number of samples in each testing group a complete NIRA employing validation tests could not be carried out. We concluded that the reflectance spectrum in the VIS-NIR-SWIR region is a promising tool for monitoring the composting process and that the composting process may provide invaluable spectral information about soil organic matter during its biochemical degradation.

Catalytic ammonia decomposition: COx-free hydrogen production for fuel cell applications

Abstract: Catalytic decomposition of ammonia has been investigated as a method to produce hydrogen for fuel cell applications. The absence of any undesirable by-products (unlike, e.g., COx, formed during reforming of hydrocarbons and alcohols) makes this process an ideal source of hydrogen for fuel cells. In this study a variety of supported metal catalysts have been studied. Supported Ru catalysts were found to be the most active, whereas supported Ni catalysts were the least active. The supports were found to play a profound role in the ammonia decomposition process. The activation energies for the ammonia decomposition process varied from 17 to 22 kcal/mol depending upon the catalyst employed. The activation energies of the supported Ir catalysts were found to be in excellent agreement with our recent studies addressing ammonia decomposition on single crystal Ir.